If its a standard 29g then it should be 30 inches wide. I would go with a size appropriate light so that you have complete coverage.
I wouldnt put the holey rock in the saltwater tank if it's been in a fw
tank, but thats just my opinion. Other may chime in if they have done it successfully.
base rock runs about $2.50 a lbs o its not a big expense. reefcleaners had 50 lbs recently for $105 with free shipping.
I currently am running dual t5's over my 125g cichlid tank, I even have the actinics going. It makes the fish colors pop, at least to me anyways. lol
But yeah, a 4 bulb t5 like theone you listed would allow you to keep a good selection of corals. If you wanted to get into anenomes and clams it might be a different story though. Individually reflected bulbs are a big plus in a marine fixture, it can make a HUGE difference. So if you are shopping around see if thats an option.